1. Field of the Invention
The present invention relates generally to generation of electricity by wind driven generators. More particularly, the present invention is a vertical axis wind turbine configured to efficiently harvest energy from the wind.
2. Related Art
Given the current global energy crisis, increased political tension with the world's largest petroleum suppliers, and the deleterious effects that fossil fuels have on the environment, renewable energy resources, including wind, are rapidly becoming less “alternative” and more an indispensable fact of life. Properly designed, a vertical-axis wind turbine features several distinct advantages over their horizontal-axis counterparts including increased safety, reduced environmental issues, and ameliorated location insensitivity to name only a few of the more conspicuous advantages.
Unfortunately, prior art vertical-axis wind turbines have suffered from fundamental design flaws, crippling their enormous latent potential. The overwhelming majority of such turbines attempt to compete with the horizontal-axis turbine's highly efficient aerodynamic lift properties by compressing oncoming wind in order to compensate with brute drag force. Others simply provide various means for reducing back pressure; a few even attempt both techniques.
U.S. Pat. No. 6,870,280 to Pechler and U.S. Pat. No. 6,309,172 to Gual disclose vertical-axis wind turbine devices that utilize both aerodynamic lift and drag forces as the principal means of inducing torque upon the rotor. However, neither of these devices employ rotor airfoils that exhibit bidirectional characteristics. Additionally, the Gual patent further teaches the exclusive use of unidirectional blades as stator blades. The primary consequence of a unidirectional blade, employed within a vertical-axis wind turbine device, is increased turbulence and muted efficiency. Moreover, due to the hook shape of Pechler's rotor blades, and the prominent bulges associated with the Gual blades, as wind exits past these blades turbulence will ensue, and laminar flow potential will be greatly diminished. Moreover, the Gual patent has stator blades that are asymmetrical, and therefore, not uniformly convex. Furthermore, the Pechler patent acknowledges that these hook shaped rotor blades are capable of imparting aerodynamic lift only when stationed at the wind receiving side of the device, and only sail propulsion elsewhere. The Pechler patent further discloses a rotor that is overwhelmingly small in relative proportion to the stator. This design attribute will inevitably produce a bow shock on the proximal side of the wind stream, substantially throttling aerodynamic efficiencies, while concurrently producing increased turbulence distally.
In U.S. Pat. No. 5,380,149 to Valsamidis, there is disclosed a vertical-axis wind turbine device. However, as concluded by the inventor, this apparatus was designed to operate as a vortex effect machine. In other words, it utilizes aerodynamic drag, or sail propulsion, as the principal means of inducing torque upon the rotor. Both the guide veins and rotor blades of this design are collectively oriented and formed such that their respective angles and various shapes combine to constrain oncoming wind to strike the concave-trailing side of the rotor blades without regard for the laminar flow of wind over their convex-leading sides. Moreover, the convex sides of the guide veins are invariably oriented opposite to the convex sides of adjacent rotor blades, and are therefore particularized exclusively for sail propulsion. Valsamidis does not teach a means for inducing a laminar flow of wind over both the convex-leading and concave-trailing sides the rotor blades in order to induce aerodynamic lift. Furthermore, Valsamidis teaches fluid flow ducts to be incorporated within all of the various blade structures. These ducts serve to equalize the air pressure on both the leading and trailing sides of the blades; thereby, aerodynamic lift potential is abandoned in favor of less efficient thrust mechanisms—aerodynamic lift is a coextensive byproduct of dissimilar air pressures that are induced upon opposite sides of an airfoil. Furthermore, placing more than three rotor airfoils on a single tier effectually closes off the center of the rotor, and a proportionate reduction in aerodynamic lift potential ultimately results.
Only one prior art device, known to the present inventors, has successfully employed both aerodynamic lift and drag forces, in concert with back pressure relief, in a consolidated vertical-axis wind turbine apparatus utilizing a multi-tiered rotor and bidirectionally primitive stator and rotor blades. U.S. Pat. No. 6,465,899 to Roberts describes an omni-directional vertical-axis wind turbine, having an increased capacity to convert wind energy to electrical energy when compared to any relative prior art apparatus, over an expanded range of wind speeds. Moreover, the aforementioned innovation does, in fact, successfully eliminate many of the drawbacks historically associated with prior art vertical-axis wind turbine designs, while concurrently facilitating a greater appreciation of the many inherent disadvantages accompanying the pervasive horizontal-axis varieties, as propounded by the aforesaid patent. However, although field testing—conducted by the principal inventor of both the referent and present invention—substantiated the referent claims, several disadvantages were discovered.
Even though the stator blades do reduce back pressure, while providing a novel means for effectively transferring torque to the rotor cups, during both their advance and return phase of rotation, they are not adequate to optimally provide this essential function omni-directionally. This is primarily a consequence of their primitive shape. These blades do not qualify as effective bidirectional airfoils, and therefore, are not conducive to the laminar conduction of wind through or around the device. Moreover, whenever the oncoming wind is oriented nearly perpendicular to the chord of the arc of the far-right stator blade—refer to FIG. 17a of the drawings—the fully cupped shape of the blade produces a significant amount of turbulence instead of channeling the wind as intended. Consequently, overall operating efficiency drops significantly, and the wind directional aggregate of the device is reduced to roughly 270 degrees out of the possible 360 degrees postulated by the patent.
The invariant hemicyclic arcs of the rotor cups do minimize drag on their convex side; however, this shape is not a paragon for either harnessing or discharging the wind as it flows through the device. Again, this is a consequence of their primitive shape. These blades are not highly efficient bidirectional airfoils, and therefore are not conducive to the laminar flow of wind through the device. Moreover, as wind moves past these rotor cups at shallow angles, the induced torque potential falls far below maximum. Peak moments of torque only occur as the oncoming wind is nearly perpendicular to the concave region of a rotor cup, and falls rapidly as the attack angles become more acute. Furthermore, because the harnessed wind cannot be released as early as would be ideal, downstream laminar flow potential is diminished.
The stator/rotor combination of this device is a very effective construct for increasing both wind speed and pressure, by means of the conservation of angular momentum, as described by the referent patent. However, their roughly equivalent structural proportion ratios attempt to compress oncoming wind well beyond the known theoretical limits of Betz Law, resulting in a bow shock on the proximal side of the device, and increased turbulence distally. This unfavorable effect occurs, to some degree, even while operating under low wind speed conditions.
Limiting the energy conversion options of the referent invention, exclusively to the transitionally-coupled generators described in U.S. Pat. No. 6,020,725 to Roberts, restricts virtually all design implementations to grid interconnections. Furthermore, requiring a plurality of these generators may be proscribed in specialized or small-scale energy output models. The foregoing critique notwithstanding, there are profound benefits to this design concept, and should not be considered fully without merit in all cases.
Additional prior art designs, developed by the present inventors, employed traditional, Darrieus-like, airfoils as stator and/or rotor blades. However, testing conclusively revealed the primary disadvantages of this paradigm to be turbulence, noise, and muted efficiency resulting from the non-bidirectional characteristics of the blades. Consequently, wind became turbulent either upon entering or exiting these developmental predecessors.